The present disclosure relates to semiconductor memory devices. More particularly, the disclosure relates to variable resistance memory devices performing a program and verification operation.
Semiconductor memory devices may be generally categorized as random access memory (RAM) and read only memory (ROM). ROM is a form of nonvolatile memory capable of retaining stored data in the absence of applied power. ROM includes programmable ROM (PROM), erasable programmable ROM (EPROM), and electrically erasable programmable ROM (EEPROM) including flash memory. Flash memory may be further classified as NOR-type flash memory and NAND-type flash memory.
RAM is a volatile memory and loses stored data in the absence of applied power. RAM may be further classified as dynamic RAM (DRAM) and a static RAM (SRAM). DRAM has been used with great commercial success for many decades, and generally stores data using a conventional capacitor element. The presence of the capacitor is what makes DRAM non volatile in its operative nature.
Emerging forms of RAM seek to replace the capacitor with other elements that are nonvolatile in their operative nature. The ferroelectric RAM (FRAM) uses a ferroelectric capacitor to store data. The magnetic RAM (MRAM) uses a tunneling magneto-resistive layer to store data and phase change memory uses the material properties of certain chalcogenide alloys to store data.
Phase change memory is a form of nonvolatile memory that interprets the phase change and corresponding resistance of a thermally alterable material as respective data states. Phase change memory may readily fabricated at low cost points using a fairly simple manufacturing process.
Phase change memory typically includes a write driver circuit capable of supplying a program current to the phase change material (e.g., GST) during a program operation. The write driver circuit typically supplies the program current in one of two states; a set current or a reset current. This program current is commonly derived from an externally provided power supply voltage (e.g., 2.5V). As conventionally understood, the set current converts the phase change material to a set state and the reset current converts the phase change material to a reset state. The set and reset states correspond to respective material properties for the phase change material. The program current is applied to the phase change material at defined levels over defined periods of time to transition between states.
In order to improve reliability of a program operation in a phase change memory, a constituent program verification operation is performed (“a program/verification operation”). Phase change memory typically performs a program/verification operation by incrementally increasing the level of the program current. Each program/verification cycle conducted at a defined program current level is termed “a program loop operation”, and a sequence of program loop operations is often used (or capable of being used) to effectively program a phase change memory.